1. Field of the Invention
The present invention relates to an operating method for continuously rolling a profiled billet to a predetermined finished cross-sectional shape of accurate dimension by means of successively arranged rolling mill stands with pairs of rolls, wherein the axes of the pairs of rolls extend perpendicularly to each other. The pairs of rolls have oppositely located pass grooves which determine the cross-sectional shape of the rolled steel section billet which travels through the pairs of rolls.
The present invention further relates to a rolling mill train for carrying out the above-described operating method.
2. Description of the Related Art
A billet emerging from a continuous rolling mill train having a diameter of, for example, approximately 10 to 25 mm, usually has tolerance deviations of .+-.0.5 mm. However, when cross-sectional shapes of accurate size are to be rolled, the tolerance deviations should only be 0.05 mm and less.
Moreover, the billet is subjected to temperature variations over its length. Because of other influences, such as, material quality, pulling and twisting between the rolling mill stands of the rolling mill train, etc., the billet emerging from the last rolling mill stand has a cross-section whose shape and behavior for further rolling cannot be exactly predetermined. This results in difficulties leading to a number of disadvantages even when rolling, for example, with a pair of rolls which form a positionally accurate, closed or box pass, wherein the preloading force or initial tension substantially exceeds the rolling force.
In the past, it was assumed that it was not possible to influence the negative properties of the billet resulting from the rolling procedure in the rolling mill train, particularly with respect to the cross-sectional shape of the billet. These difficulties are, for example, that it is not possible by controlling adjustment to change the box passes formed by the pairs of rolls. As a result, the pass may frequently not be filled completely or may be excessively filled because of the changing shape of the cross-section or because of changing material behavior, for example, due to temperature changes of the billet emerging from the rolling mill train. It has been possible to eliminate this difficulty more or less satisfactorily by using several successively arranged pairs of rolls with passes of stepped down size.
Another difficulty was the disadvantageous formation of partial, more or less strong edge fins at the circumference of the section after it emerged from the last finishing stand. It has up to now not been possible to eliminate or avoid this difficulty even with the use of several specifically arranged pairs of rolls in the above-described manner. Also, it has not been possible in the past to eliminate the disadvantages which result from the twisting tendency of the billet and the pulling between the successively arranged pairs of rolls. Finally, it has not been possible in the past to provide a shaping of the billet in the above-described type of rolling in such a way that the billet has over its entire length a uniform finished cross-sectional shape.
It is, therefore, the primary object of the present invention to improve the operating method mentioned above in such a way that the above-described difficulties and disadvantages are avoided or eliminated. In addition, a suitable arrangement for carrying out this operating method is to be provided.
In order to meet the above objects, the present invention starts from the finding that a uniform formation of the cross-sectional shape and of the structure of the billet emerging from the rolling mill train is effected by as uniform a distribution as possible of the reductions, i.e., a small increase in width and a uniform structure compacting, and by a uniform guidance of the rolling temperatures over the length of a billet and of the cross-section thereof. In rolling, the deformation occurs as a decrease in height caused by the passage of the billet through the pass. This decrease in height results in an increase of the length and an increase of the width of the billet. In rolling, the increase in length is primarily desired. The increase in width usually is a less desired result. Those cross-sectional portions which during this procedure are spread laterally, are subjected to very little or no decrease in height and, thus, are not elongated or unsatisfactorily elongated. The laterally spread portions whose height is only insignificantly decreased are taken along because of the connection between the individual cross-sectional portions and are stretched by the occurring tensional forces. The laterally spread edge portions are subjected to the danger of corner cracks. When such a lateral spreading occurs, this requires in the next rolling pass an appropriate decrease in height and, thus, increased rolling forces, increased deforming moments and an increased application power. However, the lateral spreading not only requires increased power, but it is also necessary either to have greater decreases per each rolling pass or a greater number of successively acting rolling passes must be provided.
Accordingly, in order to improve the above-described operating method, it is necessary to find a rolling deformation in which as little lateral spreading as possible occurs. The magnitude of the lateral spreading is influenced by the temperature of the rolled material, the frictional behavior between rolls and rolled material, the rolling speed, the pass shape and the roll diameters. In an operating method of this type, the temperature of the rolled material is predetermined by the rolling mill train and cannot be influenced. The frictional behavior between roll and rolled material can be advantageously influenced by an appropriate construction of the surfaces. The rolling speed is also predetermined by the rolling mill train. The pass shape is to be such that lateral spreading is essentially avoided and a reduction as uniform as possible over the width of the rolled material is achieved.
Of the above-described factors, the roll diameter is the easiest influenced in the operating method under discussion. Therefore, the diameter should be kept as small as possible, so that lateral spreading is also small and with uniform decrease in height the number of passes to be passed becomes smaller and the deformation resistance becomes smaller. Moreover, smaller rolling forces and smaller torques occur, so that the drive power may also be smaller. Furthermore, the tendency to lateral spreading is kept low, the roll wear is reduced and the desired narrow tolerances can be better maintained. It is also important that favorable deformation conditions are created in the rolling pass and that the surface of the rolled material is improved because of the short distances of contact between roll and rolled material.